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Inorg. Chem. 1996, 35, 5828-5835
Synthesis and Characterization of the Mixed-Valence Diamagnetic Two-Electron-Reduced Isopolytungstate [W10O32]6-. Evidence for an Asymmetric d-Electron Distribution over the Tungsten Sites Dean C. Duncan† and Craig L. Hill* Department of Chemistry, Emory University, Atlanta, Georgia 30322 ReceiVed February 29, 1996X The synthesis of the unprotonated two-electron-reduced isopolytungstate [W10O32]6- (2) is reported here. Additionally, full experimental details are given for the acquisition of both high signal-to-noise and integrable natural-abundance 17O NMR spectra of polyoxometalates. Titration of an acetonitrile solution containing 2 with 0.13 M triflic acid is followed by UV-visible/near-IR spectroscopy. The spectra reveal isosbestic points which are consistent with five distinct protonation states, Hx2, where x ) 0-4. Similar to a reported mixture of Hx2 species (x ) 1-2), 2 is EPR silent over the temperature range 4-300 K and is effectively diamagnetic. The 17O and 183W NMR spectra of 2 both indicate retention of the overall structure and symmetry (D4h) of the oxidized d0 precursor complex [W10O32]4- (1), and both spectra are consistent with delocalization of the two d electrons on the NMR time scale. The relative 17O electric field gradients (efg) between 1 and 2 for the same oxygen site (e.g. Oa(2)/Oa(1) for site a) were deduced from 17O line width measurements. These data are consistent with an increase in covalency for all three types of W-O-W bonds in 2 relative to 1. 183W NMR chemical shift and T1 measurements indicate that the d electron density in 2 is located primarily on the equatorial tungsten sites. Similarly, the location of the two d electrons in the reduced Wells-Dawson heteropolytungstate R-[P2W18O62]8- also is reported to be on the equatorial sites. Both 2 and R-[P2W18O62]8- are the only known diamagnetic WV-site “pure-addenda” (isometal) polyoxometalates which exhibit an asymmetric d-electron distribution over the metal sites.
Introduction d0
Polyoxometalates comprise a class of metal complexes of the group VB and VIB elements (excluding Cr) and exhibit both diverse and tunable domains of sizes, shapes, charge densities, acidities, and reversible redox potentials.1-7 Polyoxometalates in their reduced states form a potentially important class of mixed-valence compounds commonly referred to as “blues” or “heteropoly blues” owing to their intense blue color. These complexes generally are paramagnetic when reduced by an odd number of electrons and are diamagnetic when reduced by an even number of electrons. Numerous papers have been published addressing the properties of these mixed-valence species.8-45 The structure of the d0 decatungstate isopolyanion [W10O32]4(1) is illustrated in Figure 1. It consists of two defect Lindquist † Present address: Department of Chemistry, University of Texas, Austin, TX 78712 X Abstract published in AdVance ACS Abstracts, September 15, 1996. (1) References 2-7 are reviews on polyoxometalates. (2) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag: Berlin, 1983. (3) Day, V. W.; Klemperer, W. G. Science 1985, 228, 533-541. (4) Pope, M. T.; Mu¨ller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 3448. (5) Pope, M. T.; Mu¨ller, A. Mol. Eng. 1993, 3, 1-8. (6) Polyoxometalates: From Platonic Solids to Anti-retroViral ActiVity; Pope, M. T., Mu¨ller, A., Eds.; Kluwer Academic Publishers: Dordrecht, Netherlands, 1993. (7) Mu¨ller, A. J. Mol. Struct. 1994, 325, 13-35. (8) Launay, J. P. J. Inorg. Nucl. Chem. 1976, 38, 807-816. (9) Sanchez, C.; Livage, J.; Launay, J. P.; Fournier, M.; Jeannin, Y. J. Am. Chem. Soc. 1982, 104, 3194-3202. (10) Sanchez, C.; Livage, J.; Launay, J. P.; Fournier, M. J. Am. Chem. Soc. 1983, 105, 6817-6823. (11) Chemseddine, A.; Sanchez, C.; Livage, J.; Launay, J. P.; Fournier, M. Inorg. Chem. 1984, 23, 2609-2613. (12) Chemseddine, A. J. Non-Cryst. Solids 1992, 147, 313-319. (13) Kozik, M.; Hammer, C. F.; Baker, L. C. W. J. Am. Chem. Soc. 1986, 108, 2748-2749. (14) Kozik, M.; Hammer, C. F.; Baker, L. C. W. J. Am. Chem. Soc. 1986, 108, 7627-7630.
S0020-1669(96)00226-1 CCC: $12.00
([W5O14]2-) fragments linked by four corner-sharing oxygens with an unusually wide W-O-W angle of 178°.46 Each (15) Kozik, M.; Baker, L. C. W. J. Am. Chem. Soc. 1987, 109, 31593160. (16) Kozik, M.; Casan˜-Pastor, N.; Hammer, C. F.; Baker, L. C. W. J. Am. Chem. Soc. 1988, 110, 7697-7701. (17) Kozik, M.; Baker, L. C. W. J. Am. Chem. Soc. 1990, 112, 76047611. (18) Casan˜-Pastor, N.; Gomez-Romero, P.; Jameson, G. B.; Baker, L. C. W. J. Am. Chem. Soc. 1991, 113, 5658-5663. (19) Casan˜-Pastor, N.; Baker, L. C. W. J. Am. Chem. Soc. 1992, 114, 10384-10394. (20) Kozik, M.; Baker, L. C. W. In Polyoxometalates: From Platonic Solids to Anti-retroViral ActiVity; Pope, M. T., Mu¨ller, A. Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993; pp 191202. (21) Kirby, J. F.; Baker, L. C. W. J. Am. Chem. Soc. 1995, 117, 1001010016. (22) Harmalker, S. P.; Pope, M. T. J. Am. Chem. Soc. 1981, 103, 73817383. (23) Harmalker, S. P.; Leparulo, M. A.; Pope, M. T. J. Am. Chem. Soc. 1983, 105, 4286-4292. (24) Barrows, J. N.; Jameson, G. B.; Pope, M. T. J. Am. Chem. Soc. 1985, 107, 1771-1773. (25) Piepgrass, K.; Pope, M. T. J. Am. Chem. Soc. 1987, 109, 1586. (26) Piepgrass, K.; Barrows, J. N.; Pope, M. T. J. Chem. Soc., Chem. Commun. 1989, 10-12. (27) Barrows, J.; Pope, M. T. In Electron Transfer in Biology and the Solid State; King, R. B., Kurtz, D. M., Jr., Kutal, C., Norton, M. L., Scott, R. A., Eds.; Advance in Chemistry 226; American Chemical Society: Washington, DC, 1990; Chapter 21. (28) Barrows, J. N.; Pope, M. T. Inorg. Chim. Acta 1993, 213, 91-98. (29) Yamase, T. Polyhedron 1986, 5, 79. (30) Yamase, T. J. Chem. Soc., Dalton Trans. 1987, 1597. (31) Yamase, T.; Usami, T. J. Chem. Soc., Dalton Trans. 1988, 183. (32) Yamase, T.; Suga, M. J. Chem. Soc., Dalton Trans. 1989, 661. (33) Yamase, T. J. Chem. Soc., Dalton Trans. 1991, 3055. (34) Fournier, M.; Rocchiccioli-Deltcheff, C.; Kazansky, L. P. Chem. Phys. Lett. 1994, 223, 297-300. (35) Cooper, J. B.; Way, D. M.; Bond, A. M.; Wedd, A. G. Inorg. Chem. 1993, 32, 2416-2420. (36) Borshch, S. A.; Bigot, B. Chem. Phys. Lett. 1993, 212, 398-402. (37) Borshch, S. A. THEOCHEM 1995, 330, 139-143. (38) Barra, A. L.; Gatteschi, D.; Tsukerblatt, B. S.; Doring, J.; Mu¨ller, A.; Brunel, L. C. Inorg. Chem. 1992, 31, 5132-5134.
© 1996 American Chemical Society
The Two-Electron-Reduced Isopolytungstate [W10O32]6-
Figure 1. Ball-and-stick representation of [W10O32]4-, 1. W and O atoms are solid and open circles, respectively.
Lindquist fragment is composed of five tungsten-centered octahedra. Four octahedra are edge-shared within a plane (equatorial sites), and the fifth octahedron is edge-shared to all four equatorial tungsten atoms (axial site). The D4h symmetry of 1 indicates two distinct groups of tungsten centers: eight equivalent equatorial sites and two equivalent axial sites. The structure of the d0 Wells-Dawson heteropolytungstate of approximate D3h symmetry R-[P2W18O62]6- (structure not shown) consists of two defect A-PW9 fragments derived from the Keggin PW12 structure which are linked by six nearly linear W-O-W bonds.47-49 Each A-PW9 fragment consists of three axial site edge-shared octahedra which are each corner-shared to one pair of the six equatorial octahedra. The six equatorial octahedra form a loop alternately sharing edges and corners. The D3h symmetry of R-[P2W18O62]6- indicates two different groups of tungstens: twelve equivalent equatorial sites and six equivalent axial sites. In the two-electron-reduced WellsDawson heteropolytungstate R-[P2W18O62]8-, it was shown that the d electrons are located primarily on the equatorial sites as similarly concluded for the singly-reduced complex R-[P2W18O62]7-.10,13 The one-electron-reduced complex of 1, [W10O32]5-, was prepared previously by controlled-potential reduction in N,N-dimethylformamide (DMF).11 Additionally, a mixture of reduced decatungstate species of variable protonation and reduction state, [HxW10O32](y-x)- (y ) 5 and 6 for one-electron and two-electron reduction, respectively), was prepared by UV-photolysis of a single crystal of the tetraisopropylammonium salt of 1 at low temperature. On the basis of proposed superhyperfine coupling to protons in the lowtemperature ESR spectra of this photoreduced crystal, it was concluded that the d electron in [HxW10O32](5-x)- was found primarily on the equatorial sites.30 Additionally, the twoelectron-reduced complex of 1 Hx[W10O32](6-x)- (Hx2) in an uncharacterized protonation state, x, was prepared previously (39) Gatteschi, D.; Tsukerblatt, B.; Barra, A. L.; Brunel, L. C.; Mu¨ller, A.; Doring, J. Inorg. Chem. 1993, 32, 2114-2117. (40) Gatteschi, D.; Tsukerblat, B. Mol. Phys. 1993, 79, 121-143. (41) Borra´s-Almenar, J. J.; Clemente, J. M.; Coronado, E.; Tsukerblat, B. S. Chem. Phys. 1995, 195, 1-15. (42) Borra´s-Almenar, J. J.; Clemente, J. M.; Coronado, E.; Tsukerblat, B. S. Chem. Phys. 1995, 195, 17-28. (43) Borra´s-Almenar, J. J.; Clemente, J. M.; Coronado, E.; Tsukerblat, B. S. Chem. Phys. 1995, 195, 29-47. (44) Coronado, E.; Go´mez-Garcı´a, C. J. Comments Inorg. Chem. 1995, 17, 255-281. (45) Duncan, D. C.; Netzel, T. L.; Hill, C. L. Inorg. Chem. 1995, 34, 46404646. (46) Fuchs, J.; Hartl, H.; Schiller, W.; Acta Crystallogr., Sect. B: Struct. Sci. 1976, B32, 740-749. (47) Strandberg, R. Acta Chem. Scand. 1975, A29, 350-358. (48) D’Amour, H. Acta Crystallogr., Sect. B: Struct. Sci. 1976, B32, 729. (49) Dawson, B. Acta Crystallogr., Sect. B: Struct. Sci. 1953, B6, 113.
Inorganic Chemistry, Vol. 35, No. 20, 1996 5829 by controlled-potential reduction in DMF;11 however, no details on the location of the d electrons in Hx2 were given and the synthesis of unprotonated 2 has not been reported. It should be emphasized that the state of protonation in reduced polyoxometalates may have important consequences regarding their electronic structures. For example, the following observations are general for polyoxometalates with increasing protonation state: reduction potentials exhibit large positive shifts; two monoelectronic reduction waves merge to form a single bielectronic wave; and electronic absorbance bands exhibit hypsochromic shifts.8 We report here the synthesis of unprotonated 2 prepared by the two-electron electrochemical reduction of 1 in acetonitrile and its characterization by NMR (17O and 183W) and UVvisible/near-IR spectroscopy. Four distinct protonated derivatives of 2, Hx2 (x ) 0-4), are prepared in situ by addition of triflic acid. These Hx2 species are EPR silent over 4-300 K and are effectively diamagnetic. Also given are the full experimental protocols for acquisition of both high signal-tonoise and integrable natural-abundance 17O NMR spectra of polyoxometalates, which are now made possible largely through the use of high-field magnets. This ability is important since enrichment with 17O can lead to problems in the relative quantitation of oxygen sites.50 Additionally, two previously unobservable 17O NMR resonances of 1 are assigned and reported here.51 Both the 17O and 183W NMR spectra of 2 are consistent with retention of the structure of 1 and demonstrate that the two d electrons are delocalized on the NMR time scale. An analysis of 17O line widths indicates an enhanced covalence among the three types of µ2-W-O-W bridging bonds in 2 relative to those in 1. Importantly, both 183W NMR chemical shift and T1 measurements indicate that the two d electrons in 2 are located primarily on the eight equatorial sites. Consequently, 2 and R-[P2W18O62]8- are the only diamagnetic WVsite “pure-addenda” (isometal) polyoxometalates characterized to date that exhibit an asymmetric d-electron distribution among the metal sites. Experimental Section Materials. The tetra-n-butylammonium (henceforth Q) salt, QBr (Aldrich), 1.00 M QOH in methanol (Aldrich), phenolphthalein (Aldrich), and Na2WO4‚2H2O (AESAR) were commercial samples and were used as received. Triflic acid (Aldrich, 99%) was stored under dry argon and used without further purification. Acetonitrile was UVspectrophotometry grade obtained from Burdick & Jackson ( 5 mM. The digital charge-integrating mode was used for coulometry with the BAS system whereas graphical integration of current vs time plots was necessary to quantify charge using the PAR apparatus. Control experiments on electrolysis of the supporting electrolyte solution alone showed a sensitivity of the reduction potential window to the presence of small quantities of water. Consequently, all efforts were made to keep materials scrupulously dry (see Materials). The resulting H2O concentration was not measured; however, we estimate it to be
